1. Field of the Invention
The present invention relates to a thermodilution catheter, and more particularly, to a thermodilution catheter having a flexible heating filament disposed therein for applying heat to the patient""s blood for purposes of measuring cardiac output, volumetric blood flow, blood pressure, blood volume, blood components and the like.
2. Description of the Prior Art
As is well known, catheters have been developed for purposes of applying physiologic preparations directly into the blood streams of animals or humans or for measuring cardiovascular parameters such as cardiac output, blood pressure, blood volume, blood components and the like. Conventional catheters are made from various materials including plastics and are typically inserted into various body compartments, cavities and vessels to either deliver therapeutic agents, diagnostic agents, or to measure directly various physiologic parameters.
Numerous techniques have been disclosed in the prior art for measuring blood flow using catheters. For example, in U.S. Pat. No. 4,507,974, Yelderman describes a technique for measuring blood flow by applying a stochastic excitation signal to a system inlet and measuring the output signal at a downstream system outlet. The blood flow rate is then extracted by cross-correlating the excitation signal and the measured output signal. The problem addressed by systems of this type is particularly difficult since the physiologic blood vessels are elastic, thereby making classic fluid measuring techniques unacceptably inaccurate. In fact, because the blood vessels are elastic, blood flow cannot be measured unless (1) the physical heart dimensions are measured simultaneously with the blood velocity, (2) a technique is used which is independent of the vessel geometry or (3) a blood velocity technique is used which is calibrated by some other technique. Examples of each of these techniques may be found in the prior art.
For example, a prior art approach for simultaneously measuring blood velocity and vessel geometry is described by Segal in U.S. Pat. Nos. 4,733,669 and 4,869,263 and in an article entitled xe2x80x9cInstantaneous and Continuous Cardiac Output Obtained With a Doppler Pulmonary Artery Catheterxe2x80x9d, Journal of the American College of Cardiology, Vol. 13, No. 6, May 1989, Pages 1382-1392. Segal therein discloses a Doppler pulmonary artery catheter system which provides instantaneous diameter measurements and mapping of instantaneous velocity profiles within the main pulmonary artery from which instantaneous cardiac output is calculated. A similar approach is taught by Nassi et al. in U.S. Pat. No. 4,947,852. A comparable ultrasound technique is disclosed by Abrams, et al. in U.S. Pat. Nos. 4,671,295 and 4,722,347 and in an article entitled xe2x80x9cTranstracheal Doppler: A New Procedure for Continuous Cardiac Output Measurementxe2x80x9d, Anesthesiology, Vol. 70, No. 1, January 1989, Pages 134-138. Abrams et al. therein describe a technique whereby a piezoelectric ultrasound transducer is placed in the trachea of a patient in proximity to the aorta or pulmonary artery so that ultrasound waves may be transmitted toward the path of flow of blood in the artery and reflected waves received. The cross-sectional size of the artery is measured based upon the Doppler frequency difference between the transmitted and received waves. Imaging techniques such as x-ray or radio isotope methods have also been used.
Previous techniques which are geometry independent include an indicator dilution or dye dilution technique of the type first disclosed by Stewart in an article entitled xe2x80x9cThe Output of the Heart in Dogsxe2x80x9d, American Journal of Physiology, Vol. 57, 1921, Pages 27-50. Other such geometry independent techniques include a thermodilution technique as first described by Fegler in an article entitled xe2x80x9cMeasurement of Cardiac Output in Anesthetized Animals by a Thermo-Dilution Methodxe2x80x9d, Quarterly Journal of Experimental Physiology, Vol. 39, 1954, Pages 153-164 and an ionic dilution technique as described by Geddes et al. in U.S. Pat. No. 4,572,206.
On the other hand, prior art techniques for measuring blood velocity which require a secondary calibration technique include a pulse contour technique of the type described by Schreuder, et al. in an article entitled xe2x80x9cContinuous Cardiac Output Monitoring During Cardiac Surgeryxe2x80x9d, Update In Intensive Care And Emergency Medicine, Berlin: Springer-Verlag, 1990, Pages 413-416. Another so-called xe2x80x9chot wirexe2x80x9d anemometer or heated thermistor technique has been described, for example, by Tanabe, et al. in U.S. Pat. No. 4,841,981 and EP 235811 and by Sekii, et al. in U.S. Pat. No. 4,685,470 and WO 8806426.
The present invention relates to a geometry independent technique, namely, indicator dilution. In conventional indicator dilution techniques, different methods of indicator delivery may be used. For example, Khalil in U.S. Pat. No. 3,359,974 introduces indicator as a step increase and measures the resultant distal temperature change. Newbower, et al., on the other hand, discloses in U.S. Pat. No. 4,236,527 the technique of introducing the indicator as a sinusoid and measuring the distal wave attenuation. In addition, the indicator may be applied as an impulse so that the area under the resultant response may be measured as described by Normann in U.S. Pat. No. 4,576,182. Eggers, et al. in U.S. Pat. No. 4,785,823 similarly provide an impulse, but Eggers, et al. use high frequency energy to provide large heat fluxes to the blood without increasing the filament temperature. In addition, Petre describes in U.S. Pat. No. 4,951,682 an intra-cardiac impedance catheter which measures cardiac output based on changes in the electrical impedance of the blood in the right ventricle. By contrast, as described by Yelderman in the afore-mentioned U.S. Pat. No. 4,507,974, the indicator may be supplied according to a pseudo-random binary sequence and the distal response measured. Cross-correlation can then be performed between the input sequence and the output sequence, and flow is computed based upon the area under the cross-correlation curve. Each of these techniques may provide either an intermittent or a continuous measurement.
Although each of the above-mentioned techniques may use a variety of indicators, heat is the preferred indicator to be used in the clinical environment, for unlike other indicators, heat is conserved in the immediate vascular system but is largely dissipated in the periphery in one circulation time so as to eliminate recirculation and accumulation problems. On the other hand, if cold (negative heat) indicators are used, large amounts of cold may be used, for cold has relatively no deleterious effects on blood and surrounding tissues. However, when cold is used, it must be supplied in a fluid carrier such as saline since cold producing transducers are not readily economical or technically available at present. For example, such a technique is described by Webler in U.S. Pat. No. 4,819,655 and by Williams in U.S. Pat. No. 4,941,475, but the cold-based technique of Webler or Williams has significant clinical limitations in that the circulating fluid must be cooled to near ice temperature prior to input into the catheter and temperature equilibrium established, which takes a significant amount of time. In addition, the enlarged catheter segment containing the cooling elements may restrict blood flow. By contrast, if heat is used, a maximum heat infusion limitation is quickly reached since small increases in heat transducer temperature can have a deleterious effect on blood and local tissue. In fact, it can be inferred from the teachings of Ham et al. in xe2x80x9cStudies in Destruction of Red Blood Cells, Chapter IV. Thermal Injuryxe2x80x9d, Blood, Vol. 3, pp. 373-403 (1948), by Ponder in xe2x80x9cShape and Transformations of Heated Human Red Cellsxe2x80x9d, J. Exp. Biol., Vol. 26, pp. 35-45 (1950) and by Williamson et al. in xe2x80x9cThe Influence of Temperature on Red Cell Deformabilityxe2x80x9d, Blood, Vol. 46, pp. 611-624 (1975), that a maximum safe filament surface temperature is probably about 48xc2x0 C. Since the surface temperature of a heat transducer is a function of the blood flow velocity, the surface area and the heat flux, the optimum design is to maximize the heat delivered to the blood while minimizing the transducer surface temperature.
A heat transducer must satisfy several requirements if it is to be used clinically. Namely, the heat transducer or filament must be electrically safe. It also must only minimally increase the catheter cross-sectional area or diameter and must be made of materials which are non-toxic, which can be sterilized, and which can safely and easily pass through a standard introducer sheath. The heater must also be flexible so as not to increase the stiffness of the catheter body and must be capable of transferring the electrically generated heat to the surrounding blood without exceeding a safe filament surface temperature. Moreover, means must be present to continuously monitor the filament temperature to detect unsafe filament temperature and/or stagnant blood flow. However, prior art heater elements for catheters have not heretofore addressed these problems.
Prior art heater elements for thermodilution catheters have typically used simple resistive wire wound around the catheter. For example, Khalil discloses in U.S. Pat. No. 3,359,974 and Barankay, et al. disclose in an article entitled xe2x80x9cCardiac Output Estimation by a Thermodilution Method Involving Intravascular Heating and Thermistor Recordingxe2x80x9d, Acta Physiologica Academiae Scientiarum Hungaricae, Tomus 38 (2-3), 1970, Pages 167-173, wrapping the wire around the catheter but describe no methods for securing the heater material to the catheter body. Normann, in U.S. Pat. No. 4,576,182, discloses a design similar to that of Khalil. However, such exposed or minimally required wire as used in these devices increases the catheter cross-section, thereby making it difficult for the catheter to pass through an introducer and providing a rough surface which may introduce local blood clot formation. In addition, such an arrangement provides no protection from fragments of the filament becoming dislodged and does not evenly dissipate the heat, thereby producing xe2x80x9chotxe2x80x9d spots near the filament itself. It is thus desired to design a catheter heating filament which is electrically, mechanically and thermodynamically safe.
As noted above, although heat is a preferred indicator for dilution methods for measuring blood flow, the amount of heat delivered is limited or restricted by the maximal safe filament surface temperature. Although no absolute safe maximum filament temperature has been developed in the prior art, sufficient information is present in the literature to substantiate a reasonably safe maximum. For example, amongst blood, proteins and vessel tissue, red blood cells have been shown to be probably the most susceptible to higher temperatures. It is also well documented in the prior art that red blood cells can sustain an incubation temperature of 48xc2x0 C. for up to one hour before developing significant abnormalities, as described by Williamson, et al. In any event, because the actual contact time of each red blood cell flowing past the heating filament is significantly less than several seconds, such a maximum of 48xc2x0 C. is easily acceptable. A catheter heating filament can be designed to provide sufficient surface area to allow adequate heat infusion with a surface temperature below this maximum; however, changes in flow, such as sudden decreases or cardiac arrest, or changes in catheter position, such as becoming lodged against a vessel wall, may provide a local stagnant blood environment. Such stagnant environments may allow for surface temperatures which exceed these maximum limits and can thus cause harm if a method is not present for measuring filament temperature.
In addition, in prior art heating filaments for thermodilution catheters either the filament temperature has not been measured or the temperature is measured with a second thermometer. Such techniques obviously are unacceptable if a maximum safe temperature is to be maintained. Acceptable temperature sensing materials require a sufficiently high temperature coefficient of resistance to measure changes in temperature. Compositions of this type are described, for example, by Morris, et al. in an article entitled xe2x80x9cThin Film Thermistorsxe2x80x9d, Journal of Physics Engineering: Scientific Instruments, Vol. 8, 1975, Pages 411-414. It is desired to develop a method for continuously measuring the filament temperature without the use of a secondary measuring transducer such as a thermistor or thermocouple of the type used in these prior art devices.
A classical prior art method of measuring fluid velocity uses a hot-wire anemometer. In accordance with this technique, a filament is heated with a constant power or at a constant heat flux and the resistance is measured. If a filament with a high temperature coefficient of resistance is used, the measured resistance can be used to directly calculate filament temperature, for the filament temperature is monotonically and inversely proportional to the fluid velocity. Such a technique has been previously described as a means for measuring blood velocity by Gibbs in an article entitled xe2x80x9cA Thermoelectric Blood Flow Recorder in the Form of a Needlexe2x80x9d, Proc. Soc. Exp. Biol. and Med., Vol. 31, 1933, Pages 141-146. However, as noted by Gibbs in that article, such a technique has been limited to peripheral vessels and cannot give absolute blood volumetric flow rates, only velocity. It is desired to adapt such techniques to thermodilution measurements to prevent localized overheating of the blood.
As noted above, previous heating elements for thermodilution catheters have generally used wire and wrapped it around the catheter. However, such an approach provides for uneven heat densities on the catheter since the wire tends to be hot and the space between the wire cooler. A more uniform material is desired which allows for more even heat densities and the elimination of xe2x80x9chotxe2x80x9d spots. This is not possible with wire, for wire, even very small gauge wire, provides a larger cross-section for the catheter than necessary since there is unused space between the wire even when the wire is wound very compactly. A more uniform material would allow for a better distribution of the same quantity of heat with only a small increase in catheter cross-section. It is thus desirable to develop a filament material which minimally increases a cross-sectional area of the catheter and which provides a more uniform filament heat flux.
Accordingly, there are numerous problems with prior art thermodilution catheters which render them either inaccurate or unacceptably unsafe for use in the clinical environment. There is thus a long-felt need in the art for a filamented thermodilution catheter which overcomes these limitations of the prior art so as to allow production of a safe, accurate thermodilution catheter. The present invention has been designed to meet this need.
A safe, accurate thermodilution measurement may be made in accordance with the present invention by using a catheter having a heating filament which resides internal to the catheter body, either in a preformed catheter lumen or beneath an outer sheath, and which preferably does not directly contact the blood. The heating filament is preferably made of a thin, flexible material which may be wrapped either on the exterior of the catheter body wall and then covered by an external sheath material so that the heating filament material is not exposed to the blood or on the outer surface of a supporting sheath inserted into the catheter lumen. The heating filament so designed supplies a quantity of heat to the flowing blood which is used for measuring the volumetric blood flow using an indicator dilution equation. During use, the filament temperature is preferably measured simultaneous with the thermodilution measurement without the use of a second measuring transducerxe2x80x94not to calculate velocity, but so that a safe filament temperature may be maintained. This is accomplished in a preferred embodiment by forming the heating filament of a material which has a resistance proportional or inversely proportional to its temperature.
In addition, the invention allows determination of blood flow using thermodilution techniques in clinical situations which were impossible using the classic bolus thermodilution method. For example, in applications such as measuring left ventricular blood flow or measuring hepatic blood flow, the natural blood flow is xe2x80x9cretro gradexe2x80x9d, i.e., the blood flows from the distal end of the catheter toward the proximal portion. In this situation, it is necessary to place the thermal filament at the distal tip and locate the detection thermistor or thermocouple in a proximal location. The classical thermodilution catheter cannot be used in such a case since it is necessary to have the indicator flow under and past the detection thermistor or thermocouple, which in the case of xe2x80x9cretro gradexe2x80x9d blood flow induces an enormous amount of cross-talk and error. However, if an electrical filament is used, such as in the present invention, the electrical current can be passed under and past the detection thermistor or thermocouple without inducing thermal cross-talk and error.
Thus, in accordance with the invention, a thermodilution catheter is provided which comprises:
a flexible tubular catheter member adapted for introduction into a blood vessel of a patient;
a flexible heating filament disposed with respect to the catheter member so as not to touch the patient""s blood when the catheter member is inserted into a blood vessel, the heating filament applying a predetermined quantity of heat to blood in the blood vessel; and
temperature detecting means for detecting downstream temperature variations of the blood as a result of application of the predetermined quantity of heat to the blood.
In accordance with a preferred embodiment of the invention, the catheter member comprises a substantially cylindrical body wall portion and an outer sheath, where the heating filament is wrapped in a thin layer about the body wall portion of the catheter member and enclosed by the sheath. The sheath may be a flexible material formed by extrusion or blow molding or a flexible material which shrinks to form-fit the heating filament and the body wall portion when a sufficient quantity of heat is applied thereto. A layer of material with high thermal conductivity may also be disposed about the heating filament so as to create a more uniform surface temperature. The body wall portion may also have a reduced diameter in the region where the heating filament is wrapped therearound such that the resulting total diameter in that region is approximately equal to the diameter of the body wall portion in other regions. In some embodiments, the sheath material may actually contain the resistive heating material so as to form one element, thereby eliminating the need for a separate filament and sheath.
The heating element is placed upstream or downstream from the temperature detecting means depending upon whether xe2x80x9cretro gradexe2x80x9d or flow-directed measurements are made. Also, at least one set of electrical leads is attached to the heating filament at a first end thereof so as to apply power to the heating filament and is attached at a second end thereof to a cardiac output computer which applies an appropriate power signal to the electrical leads when the predetermined quantity of heat is to be applied to the blood. Typically, the cardiac output computer continuously monitors both current and voltage delivered to the catheter filament. From the product of current and voltage, the delivered heat may be computed, and from the ratio of voltage to current, the actual filament temperature may be calculated.
In accordance with another aspect of the invention, calibrating means are provided for calibrating both the heating filament and the temperature detecting means. Preferably, the calibrating means comprises a Read Only Memory (ROM) contained within the catheter member for storing calibration information for at least one of the heating filament and the temperature detecting means, as well as any other necessary information. Such calibration information may further include heating filament resistance at a given temperature, heating filament heat transfer efficiency, temperature coefficient of resistance and thermistor information. Moreover, the ROM may be connected to a cardiac output computer so as to pass a program segment, stored in the ROM, of a program used by the cardiac output computer to calculate cardiac output of the patient, whereby calculation of the patient""s cardiac output cannot commence until the cardiac output computer is connected to the ROM and the program segment transferred to the cardiac output computer.
In accordance with another preferred embodiment of the catheter apparatus of the invention, the catheter member has at least one lumen through which the heating filament is removably inserted. In such an embodiment, the heating filament is wrapped in a thin layer about a substantially cylindrical supporting member such that the combination has an outer diameter which is approximately equal to an inner diameter of the lumen through which it is inserted. Preferably, the heating filament is approximately 5 to 10 centimeters in length and is disposed approximately 10 centimeters from the temperature detecting means.
In accordance with another aspect of the invention, the heating filament is comprised of a material having a high temperature coefficient of resistance, whereby resistance of the heating filament is inversely proportional to its temperature (i.e., it has a negative temperature coefficient of resistance). This aspect of the invention enables power to the heating filament to be reduced when resistance of the heating filament exceeds a predetermined resistance amount, which is reached when the temperature of the heating filament reaches approximately 52xc2x0 C. A material suitable for the heating filament thus has a temperature coefficient of resistance greater than 0.001 xcexa9/xcexa9-xc2x0 C. Also, such a material preferably has a low thermal capacitance and high thermal conductivity. Preferred heating filament materials include an alloy of approximately 70% nickel and 30% iron and an alloy of approximately 29% nickel, 17% cobalt and 54% iron.
The invention further relates to a method of applying heat to blood in a blood vessel of a patient for purposes of conducting a thermodilution measurement. Such a method in accordance with the invention comprises the steps of:
inserting a flexible tubular catheter member having a heating element disposed beneath an outer sheath thereof into the blood vessel of the patient so as not to directly expose the heating element to blood in the blood vessel;
applying power to the heating element so as to generate a predetermined quantity of heat; and
applying the predetermined quantity of heat through the catheter member to blood in the blood vessel.
A preferred embodiment of such a method in accordance with the invention preferably comprises the further steps of:
forming the heating filament of a material having a high temperature coefficient of resistance whereby resistance of the heating filament is proportional or inversely proportional to its temperature and having a low thermal capacitance and a high thermal conductivity; and
reducing power to the heating filament when resistance (and consequently the temperature) of the heating filament exceeds a predetermined resistance amount (temperature). These steps prevent localized overheating and thus help assure patient safety.
Another embodiment of a method in accordance with the invention for applying heat to blood in a blood vessel of a patient for purposes of conducting a thermodilution measurement comprises the steps of:
inserting a flexible tubular catheter member into the blood vessel of the patient;
removably inserting a flexible heating filament through a lumen of the catheter member;
applying power to the heating filament so as to generate a predetermined quantity of heat; and
applying the predetermined quantity of heat through the catheter member to blood in the blood vessel.
Such methods in accordance with the invention also preferably comprise the further step of calibrating the heating filament using a memory disposed at a proximal end of the catheter member, where the memory stores calibration information including heating filament resistance at a given temperature and/or heating filament heat transfer efficiency and thermistor information. This memory is then connected to a cardiac output computer so that the cardiac output calculation can use the calibration information stored in the memory.
Pirating of the catheter of the invention may also be prevented by performing the thermodilution measurement in accordance with the following steps:
inserting a flexible tubular catheter member having a flexible heating filament disposed beneath an outer sheath thereof into a blood vessel of a patient such that the heating filament does not directly contact the patient""s blood;
applying power to the heating filament so as to generate a predetermined quantity of heat;
applying the predetermined quantity of heat through the catheter member to blood in the blood vessel;
detecting downstream temperature variations of the blood as a result of application of the predetermined quantity of heat to the blood;
storing in a memory disposed at a proximal end of the catheter member a program segment of a program used by a cardiac output computer to calculate cardiac output of the patient;
connecting the memory to the cardiac output computer;
transferring the program segment to the program of the cardiac output computer; and
calculating the cardiac output of the patient in accordance with the program including the program segment and the detected temperature variations.
Thus, by providing a heating filament which is minimally thin, heat from the filament may be transferred to the surrounding blood in accordance with the invention without significantly increasing the overall cross-sectional area of the catheter. Also, by providing a surface coating for the resulting catheter which has no rough areas to induce blood clot formation, the thermodilution catheter of the invention may be more safely used in clinical settings. Moreover, by monitoring the heating filament temperature, localized overheating may be prevented. Other elements or drugs, such as heparin, antibiotics and the like may be added to the filament sheath to reduce the possibility of any complications as a result of the measurements using the catheter of the invention.